Flip-chip semiconductor packages employ advanced packaging technology that is characterized by mounting a semiconductor chip in a face-down manner on a substrate and electrically connecting the semiconductor chip to the substrate via a plurality of solder bumps. This structure yields significant benefits without having to use relatively space-occupying bonding wires for electrically connecting the semiconductor chip to the substrate, thereby making the overall package structure more compact in size.
Referring to FIG. 1, for forming a solder bump 150 to a semiconductor chip 100, the first step is to form an under bump metallurgy (UBM) structure 130 on a bond pad 110 of the semiconductor chip 100. The UBM structure 130 includes an adhesion layer 130a such as aluminum layer formed over the bond pad 110; a barrier layer 130b such as nickel/vanadium (Ni/V) alloy applied over the adhesion layer 130a; and a wetting layer 130c such as copper layer formed on the barrier layer 130b. A solder material can be applied over the wetting layer 130c and reflowed to form the solder bump 150 on the UBM structure 130. This UBM structure 130 serves as a diffusion barrier and provides proper adhesion between the solder bump 150 and the bond pad 110 of the semiconductor chip 100.
Fabrication of the UBM structure generally adopts sputtering, evaporation and plating processes.
FIGS. 2A to 2E illustrate conventional fabrication processes for a solder bump on a flip chip. Referring to FIG. 2A, the first step is to prepare a semiconductor chip 100 formed with a plurality of bond pads 110 on a surface thereof, and to apply a passivation layer 120 over the surface of the semiconductor chip 100. The passivation layer 120 is selectively removed to expose the bond pads 110 of the semiconductor chip 100. Then, sputtering and plating processes are performed to form a UBM structure 130 on each of the bond pads 110.
Referring to FIG. 2B, next, a solder mask film 140 such as dry film is applied over the passivation layer 120 and formed with a plurality of openings 141 for exposing the UBM structures 130.
Referring to FIG. 2C, then a solder-applying process is performed by which a solder paste such as tin/lead (Sn/Pb) alloy is applied via the openings 141 through the use of screen-printing technology over the UBM structures 130 to form a plurality of solder bumps 150 respectively on the UBM structures 130.
Referring to FIG. 2D, a first reflow process is carried out to bond the solder bumps 150 to the corresponding UBM structures 130. Then, the solder mask layer 140 is removed, and a second reflow process is performed to make the solder bumps 150 have a ball shape, as shown in FIG. 2E.
Prior art references relating to UBM technology include, for example, U.S. Pat. No. 5,773,359 entitled “Interconnect System and Method of Fabrication”, U.S. Pat. No. 5,904,859 entitled “Flip Chip Metallization”, and U.S. Pat. No. 5,937,320 entitled “Barrier Layers for Electroplated SnPb Eutectic Solder Joints”; to name just a few.
In respect of fabricating a UBM structure on an aluminum-made bond pad (hereinafter referred to as “aluminum pad”) of a semiconductor chip, an aluminum layer (or a chromium layer) is firstly formed over the aluminum pad to provide adhesion between the aluminum pad and the UBM structure. Then, a nickel/vanadium (Ni/V) layer is deposited over the aluminum layer to serve as a barrier for preventing intermetallic compounds formed from reaction between the aluminum pad and a solder-bump electrode. Finally, a copper layer (or a layer made of nickel, palladium or molybdenum) is applied on the Ni/V layer for allowing the solder bump to be successfully bonded to the UBM structure. However, this UBM structure is not applicable to a copper-made bond pad (hereinafter referred to as “copper pad”) because the aluminum layer formed over the bond pad has relatively poor adhesion to copper, making the UBM structure not strongly bonded to the copper pad. Therefore, for forming a UBM structure on a copper pad, the first step is to apply a titanium (Ti) layer over the copper pad, and the Ti layer provides good adhesion between the UBM structure and the copper pad. Then, a Ni/V layer and a copper layer are formed over the Ti layer for allowing a solder bump to be strongly bonded thereon. Although the Ti layer enhances adhesion between the copper pad and the UBM structure, this Ti layer is poorer in electrical conductivity than aluminum and thus degrades electrical connection between the solder bump and the copper pad.
Therefore, the problem to be solved herein is to provide good electrical connection and adhesion between a UBM structure and a copper-made bond pad.